U.S. Pat. No. 4,387,886 discloses a vertically adjustable device of the kind set forth in which the support tube is arranged within a longitudinally slit guide tube which is embraced by the semicylindrical shells and which is reduced in diameter when the shells are clamped together so that the support tube can be fixed in any vertical position. The closed lower end or bottom of the guide tube forms the abutment for the lower end of the spring means. In order to obtain an easy vertical adjustment or movement of the support tube when the clamping means formed by the shells is loosened, and to exert a great clamping force without excessive manual force it is necessary that relatively close tolerances must be observed between the inner diameter of the semicylindrical shells and the outer diameter of the guide tube and between the inner diameter of the guide tube and the outer diameter of the support tube. The necessity to maintain relatively close tolerances between three cooperating parts renders the manufacture of this device comparatively expensive.